The early Macs used ADB (Apple Desktop Bus), which uses a 4-pin mini-DIN as its physical connector. PCs use PS/2, which uses a 6-pin mini-DIN. They're not compatible at all.
That's what API design is so important, and so difficult. By analogy the DIN connector is simply calling convention and ADB the actual function calls.
Now, if you just wanted to hack peripheral hardware at the time, you would have invested in a lot cheaper (expendable) computer with a standard serial / parallel port, so I don't really understand what you are saying.